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Article Electrical Properties of / Compounds for High- Insulation

Sameh Ziad Ahmed Dabbak 1, Hazlee Azil Illias 1,* ID , Bee Chin Ang 2, Nurul Ain Abdul Latiff 1 and Mohamad Zul Hilmey Makmud 1,3,* ID

1 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia; [email protected] (S.Z.A.D.); [email protected] (N.A.A.L.) 2 Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia; [email protected] 3 Complex of Science and Technology, Faculty of Science and Natural Resources, University Malaysia Sabah, Kota Kinabalu 88400, Malaysia * Correspondence: [email protected] (H.A.I.); [email protected] (M.Z.H.M.); Tel.: +60-3-7967-4483 (H.A.I.)  Received: 12 April 2018; Accepted: 30 May 2018; Published: 4 June 2018 

Abstract: In high-voltage insulation systems, the most commonly used material is polymeric material because of its high strength, high resistivity, and low in addition to good chemical and mechanical properties. In this work, various polymer compounds were prepared, consisting of low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), HDPE/PP, and LDPE/PP polymer blends. The relative and breakdown strength of each sample types were evaluated. In order to determine the physical properties of the prepared samples, the samples were also characterized using differential scanning calorimetry (DSC). The results showed that the dielectric constant of PP increased with the increase of HDPE and LDPE content. The breakdown measurement data for all samples were analyzed using the cumulative probability plot of Weibull distribution. From the acquired results, it was found that the dielectric strengths of LDPE and HDPE were higher than that of PP. Consequently, the addition of LDPE and HDPE to PP increased the breakdown strength of PP, but a variation in the weight ratio (30%, 50% and 70%) did not change significantly the breakdown strength. The DSC measurements showed two exothermic crystallization peaks representing two crystalline phases. In addition, the DSC results showed that the blended samples were physically bonded, and no co-crystallization occurred in the produced blends.

Keywords: insulation materials; ; dielectric characterization; high-voltage engineering

1. Introduction Thermoplastic polymers, such as polyethylene (PE), polypropylene (PP), (PVC), and (PS) are the main products of industrial engineering, which are employed in several electrical application fields. In high-voltage equipment, polymers are commonly used in the insulation systems because of their exceptional mechanical, electrical, chemical, and thermal properties and their low production cost. In insulation materials, which are produced by mixing different polymers at different weight ratios, polymers are used to modify the physical and mechanical properties of the final products, maintaining a lower production cost. High-density polyethylene (HDPE), low-density polyethylene (LDPE), and PP are the most widely used polymers in the world market, employed for electrical equipment, automotive engineering material, and packaging. The most useful properties of PE and PP are oil resistance, rigidity, good

Energies 2018, 11, 1448; doi:10.3390/en11061448 www.mdpi.com/journal/energies Energies 2018, 11, 1448 2 of 13 stiffness, and thermal stability [1]. Conversely, the utilization of PP and PE is restricted in certain areas owing to some weak properties such as melt viscosity. The weak properties of LDPE include a lack of mechanical and thermal resistance. Hence, numerous researchers are working on improving the properties of LDPE by mixing it with other polymers that have high temperature resistance [2,3]. In addition, the poor impact strength of PP and its weak Young’s modulus restrict its applications. These poor properties may be enhanced by mixing PP with PE [4–7]. In these blends, the impact and tensile strength are improved [8,9]. The polymer blends also compromise significant processing benefits in the melting polymer fraction [10]. The breakdown strength attracted major interest in early polymer studies. In 1955, the dielectric strength of insulation materials and its relation with the electric field strength between high voltage and ground were studied. It was found that by increasing the internal ionization through an increase of the electric field strength, an avalanche would form, leading to insulation breakdown if the electric field reached a very high value. The insulation breakdown is also caused by the expansion of in polymers, particularly in LDPE, which affects the material, wrecking its formation. Essentially, the electrical trees are instigated when the material is exposed to excessive electrical field stress over long periods of time. Defects occur, which are triggered by electrical trees, that further deteriorate the material dielectric strength, upsurge the electrical stress, and speed up the (PD) process [11]. The tracking resistance was studied for linear low-density polyethylene and natural rubber LLDPE/NR blends with different weight ratios by measuring carbon track development and leakage current. It was found that the mixture of 80:20 LLDPE/NR was a better blend because of its lowest degradation [12]. The breakdown strength and stress tracking measurements were carried out for blends containing different ratios of recycled HDPE in resins with HDPE. It was revealed that the dielectric strength was not good for the recycled blends in comparison to the pure material because of the existence of conductive impurity [13]. These studies focused on the factors engaged in a breakdown mechanism. These factors include the dielectric properties, physical structure and deformity, additives and pollution, and space charge injection. The mechanism that leads to breakdown is still unclear, despite it is well known that breakdown is a result of conducting channels crossing the insulation material. A low electrical conductivity in an insulation material is required to avoid the possibility of breakdown. It depends on the amount and the concentration of charge carriers, the charge of the charge carriers, and their mobility. The direct current (DC) conductivity of polymer blends containing HDPE and LDPE was measured. It was found that the blend of LDPE with 5% HDPE had a reduced conductivity [14]. Although many studies on dielectric strength of pure polymer materials with micro- or nanoparticles have been carried out, the electrical properties, especially for high-voltage applications of polymer blends, are limited. Most of the polymer blends studies were performed on the blends mechanical and physical properties. However, it is also essential to assess the breakdown strength of blended materials in order to investigate their capability to be used in high-voltage insulation systems. Therefore, in this work, the breakdown strength of different blends of HDPE/PP and LDPE/PP was investigated. Weibull distribution was plotted to determine the probability occurrence and to analyze alternating current (AC) breakdown strength. The data were used to evaluate the dielectric strength of the blended compounds compared to the pure samples. In addition, the dielectric responses of polymer blends as a function of were compared. Differential scanning calorimetry (DSC) was used on all prepared blended samples to characterize their thermal properties, such as the amount of crystallinity and the melting temperature.

2. Samples and Experimental Process

2.1. Sample Preparation The materials used in this work, high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP), were obtained from Sigma Aldrich (St. Louis, MO, USA) with Energies 2018, 11, 1448 3 of 13 product numbers of 428019, 428043 and 427888, respectively. From the product specification sheets, the melt flow index of HDPE, LDPE, and PP are 42, 25, and 12 g/10 min respectively, and the density Energies 2018, 11, x FOR PEER REVIEW 3 of 13 3 is 0.947, 0.92 and 0.9 g/cm , respectively. The average molecular weight Mw of PP is about 250,000. 3 Theis required 0.947, 0.92 samples and 0.9of g/cm pure, respectively. polymers,in The addition average to molecular the HDPE/PP weight and LDPE/PPof PP is about blends 250,000. with weightThe ratiosrequired of 3:7, samples 5:5, and of 7:3,pure were polymers, produced in addi for eachtion to mixture. the HDPE/PP The mixed and specimens LDPE/PP wereblends marked with as HP1,weight HP2, ratios HP3, of 3:7, and 5:5, LP1, and LP2, 7:3, LP3were for produc HDPE/PPed for each and LDPE/PP,mixture. The respectively. mixed specimens The selection were marked on the mixingas HP1, ratio HP2, of HDPE/PP HP3, and LP1, and LDPE/PPLP2, LP3 for blends HDPE/P wasP basedand LDPE/PP, on past worksrespectively. [2,4,8,15 The–18 selection]. The materials on the weremixing mixed ratio mechanically of HDPE/PP using and LDPE/PP a twin screwblends extruder,was based with on past a 150 works rpm [2,4,8,15–18]. screw rotating The materials speed for 5 min.were Themixed barrel mechanically extruder using temperature a twin screw was setextr fromuder, 140with to a 180150 ◦rpmC from screw the rotating hopper speed to the for die.5 Thinmin. samples The barrel with extruder 150–250 temperatureµm thickness was were set from prepared 140 °C using to 180 an °C adjustable from the hopper temperature to the die. hydraulic Thin press.samples The temperature with 150–250 ofµm the thickness stainless were steel prepared mold, whichusing an was adjustable covered temperature by aluminum hydraulic foil, was press. kept fromThe 170 temperature to 180 ◦C of with the astainless pressure steel of mold, 35 bar which for 10 was min covered to grant by aluminum complete melting.foil, was kept Slow from cooling 170 was°C carried to 180 out°C with by turning a pressure off of the 35 heater bar for and 10 min waiting to grant for complete the sample melting. to reach Slow the cooling room temperature.was carried Then,out the by sampleturning wasoff the removed heater fromand waiting the mold. for the sample to reach the room temperature. Then, the sample was removed from the mold. 2.2. Breakdown Experiment 2.2. Breakdown Experiment The schematic diagram of the breakdown experiment setup is shown in Figure1. The breakdown The schematic diagram of the breakdown experiment setup is shown in Figure 1. The test and systems were designed following ASTM D-149-97a standards. The test chamber breakdown test and electrode systems were designed following ASTM D-149-97a standards. The test consisted of two cylindrical stainless-steel electrodes of 25 mm diameter. The specimen was located chamber consisted of two cylindrical stainless-steel electrodes of 25 mm diameter. The specimen was between the two electrodes. The upper electrode was connected to a high-voltage supply, while the located between the two electrodes. The upper electrode was connected to a high-voltage supply, bottom electrode was connected to the ground. The electrodes and specimen were fully immersed while the bottom electrode was connected to the ground. The electrodes and specimen were fully in siliconeimmersed oil in to silicone avoid discharges oil to avoid and discharges flashover and on flashover the sample on surfacesthe sample during surfaces voltage during application, voltage priorapplication, to material prior breakdown. to material To breakdown. remove the To unwanted remove the bubbles unwanted which bubbles might which affect might the test affect results, the thetest results, chamber the was test placedchamber inside was placed a inside oven a vacuum at room oven temperature at room temperature for 20 min eachfor 20 time min theeach oil wastime changed. the oil was changed. TheThe applied applied voltage voltage across across the the test test electrodes electrodes was was increasedincreased fromfrom zero until breakdown breakdown of of the the samplesample occurred. occurred. The The voltage voltage rate rate increment increment was was 500 500 V/s. V/s. WhenWhen a breakdown happened, happened, the the voltage voltage sourcesource was was disrupted disrupted accordingly. accordingly. The The test test was was repeatedrepeated forfor ninenine times on on new new samples samples of of each each materialmaterial type. type. In In order order to to obtain obtain accurateaccurate breakdow breakdownn voltage voltage results, results, the the thickness thickness of the of thesamples samples on on thethe breakdownbreakdown spot was measured using a micromet micrometer.er. This This was was because because the the specimen specimen thickness thickness couldcould not not be fullybe fully uniform. uniform. The The insulating insulating oil oil was was replaced replaced after after each each material material was was tested tested to to reduce reduce the influencethe influence of changes of changes of the oil of quality the oil onquality the test on resultsthe test due results to contaminants, due to contaminants, which were which generated were aftergenerated each sample after waseach tested. sample was tested.

HV electrode

3.2 mm round edge 25 mm diameter

120 mm

Silicone oil

Ground electrode

100 mm

FigureFigure 1. 1.Breakdown Breakdown measurement measurement chamber.chamber.

The breakdown strength was measured by dividing the recorded by the sample thickness, using:

Energies 2018, 11, 1448 4 of 13

The breakdown strength was measured by dividing the recorded breakdown voltage by the sample thickness, using: EB = VB/d (1) where VB is the breakdown voltage, and d is the sample thickness at the point in which the breakdown occurs. The Weibull probability distribution was used to describe statistically the breakdown strength distribution data. Equation (2) was used to calculate and plot the Weibull probability parameters and curves, which symbolize the breakdown strength probabilities at any random value x, defined as:

F(x) = 1 − exp(−x/α)β (2) where β is the shape parameter correlated to the scattering of the data and shows the degree of failure rate. The EB curve distribution is narrower when β is higher. The shape parameter increases when the dielectric behavior reliability is increased. The scale parameter α represents the voltage value at which a 63.2% probability of breakdown occurs [19,20]. It was used to compare the breakdown strength between different insulation types. Weibull distributions with a shape parameter less than 1 have a failure rate that decrease with the variable x, while Weibull distributions with a shape parameter greater than 1 have a failure rate that increases with x.

2.3. Physical Characterization Using Differential Scanning Calorimetry (DSC) The crystallinity evaluations of the HDPE/PP and LDPE/PP samples were carried out using Mettler DSC 820 (Mettler Toledo, Columbus, OH, USA). DSC is considered the best technique for examining the thermal properties of polymers, the amount of crystallinity, the melting temperature, and the transition temperature of a polymer. Because of the effect of the degree of crystallinity on the dielectric properties of polymers, DSC measurements were performed [21]. Measuring the differences in the heat flow between the reference and the sample to maintain the same increment of temperature among them is the basic principle of DSC. Around 10 mg of sample was used for each test. For all DSC inspections, a temperature range of 30 ◦C to 190 ◦C was selected, with a linear increment–decrement rate of 5 ◦C/min at ambient atmosphere. The samples were first heated from 30 ◦C to 190 ◦C and then cooled down to 30 ◦C. After that, the samples were reheated again to 190 ◦C.

2.4. Impedance Measurement Electrochemical impedance spectroscopy (EIS) was used for the impedance measurements. EIS was operated at 1 Vpp test voltage with a frequency range of 100 kHz to 40 Hz. The purpose of the dielectric measurements is to compare the dielectric response between different material types, which can be shown clearly within this frequency range at a low voltage level. The impedance measurements were performed at room temperature and atmospheric pressure. Two stainless-steel electrodes of equal size were designed, with 10 mm thickness and 35 mm diameter. The two electrodes were placed on the top and bottom of the sample, where the top and bottom electrodes were set as working and ground electrodes, respectively. The test sample was located between the working electrode and the ground electrode. In order to eliminate the stray current, a ring guard electrode was placed on the surface of the sample around the working electrode. The real component Z0 and the imaginary component Z00 were recorded by EIS at different applied . The Z impedance expression is given by:

Z = Z0 + jZ00 (3)

The permittivity is defined as: Energies 2018, 11, 1448 5 of 13

0 00 εr = εr + jεr (4)

0 00 where εr is the real part of the permittivity, and εr is the imaginary part of the permittivity εr. The permittivity components are estimated by:

00 0 Z Energies 2018, 11, x FOR PEER REVIEW ε = 5 of 13 r 2 (5) 2π f C0Z

′0 ɛ = Z (6) ε 00 = (6) r 22 2π f C0Z wherewhere ff isis the the frequency frequency of of the the electric , field, while while the the free free space space capacitance between between the the electrodes electrodes is represented by . The energy from an external electric field amassed within a material is defined is represented by C0. The energy from an external electric field amassed within a material is defined byby the the real real permittivity permittivity value, value, and and how how dissipative dissipative th thee material material is is against against an an ex externalternal electric electric field field is is describeddescribed by by the the imaginary imaginary permittivity permittivity [22–24]. [22–24].

3.3. Results Results and and Discussion Discussion InIn this this section, section, the the outcomes outcomes from from the the measurem measurementsents of the breakdown breakdown st strengthrength of the HDPE/PP andand LDPE/PP LDPE/PP compounds compounds are are presented. presented. In In addition, addition, pure pure HDPE, HDPE, LDPE, LDPE, and and PP PP samples samples were were examined,examined, and and the the results results were were compared compared with with those those obtained obtained from from the compound the compound samples. samples. The cumulativeThe cumulative probability probability plot of plot Weibull of Weibull distribution distribution and the and average the average breakdown breakdown were used were to analyze used to theanalyze breakdown the breakdown measurement measurement data. data.

3.1.3.1. Breakdown Breakdown Strength Strength FigureFigure 22a,ba,b show the average breakdown strength calculated from the experimental results data ofof the the HDPE/PP HDPE/PP and and LDPE/PP LDPE/PP samples, samples, respectively. respectively. It Itis isclear clear that that the the polyethylene polyethylene samples samples had had a highera higher dielectric dielectric strength strength than than the polypropylen the polypropylenee samples. samples. The average The average breakdown breakdown of HDPE, of LDPE, HDPE, andLDPE, PP, and as shown PP, as shownin these in figures, these figures, were 70, were 79, 70, and 79, 55 and kV/mm, 55 kV/mm, respectively. respectively. It can Itbe can noticed be noticed that therethat therewas an was increment an increment in the in breakdown the breakdown strength strength of PP of when PP when it was it compounded was compounded with withLDPE. LDPE. The breakdownThe breakdown strength strength of LP1, of LP1,LP2, LP2,and LP3 and LP3were were 57, 62, 57, and 62, and63 kV/mm, 63 kV/mm, respectively. respectively. In addition, In addition, the averagethe average breakdown breakdown strength strength of the of thePP PPsamples samples was was also also improved improved when when they they were were mixed mixed with with HDPE,HDPE, resulting resulting in in 59, 59, 64, 64, and and 67 kV/mm for for HP1, HP1, HP2, HP2, and and HP3 HP3 respectively. respectively. The The mixture mixture of of PP PP containingcontaining HDPE HDPE or or LDPE LDPE showed showed enhanced enhanced brea breakdownkdown of of PP. PP. However, However, despite despite the the improvements, improvements, thethe breakdown breakdown strengths strengths were were still still lowe lowerr than than those of pure HDPE and LDPE.

80 80

60 60

40 40

20 20

0 0 PP HDPE HP1 HP2 HP3 PP LDPE LP1 LP2 LP3 Breakdown strength (kV/mm) strength Breakdown Breakdown strength (kV/mm) strength Breakdown Sample type Sample type (a) (b)

FigureFigure 2. 2. AverageAverage breakdown strength ofof ((aa)) High-densityHigh-density polyethylene (HDPE)/polypropylene (HDPE)/polypropylene (PP) (PP) blends;blends; ( (bb)) Low-density Low-density polyethylene polyethylene (LDPE)/polypropylene (LDPE)/polypropylene (PP) (PP) blends. blends.

The occurrence probability plots versus voltage of the measured breakdown strength values for the HDPE blend containing PP and for the LDPE blend containing PP are shown in Figure 3a,b, while Tables 1 and 2 show the scale and shape parameters found for all tested samples. It can be seen that the breakdown strength spreads over a wide range of values for the same insulation material. The shape parameter β for all results shows values higher than 1 due to the fact that the probability of breakdown occurrence increases with the applied voltage.

Energies 2018, 11, 1448 6 of 13

The occurrence probability plots versus voltage of the measured breakdown strength values for the HDPE blend containing PP and for the LDPE blend containing PP are shown in Figure3a,b, while Tables1 and2 show the scale and shape parameters found for all tested samples. It can be seen that the breakdown strength spreads over a wide range of values for the same insulation material. The shape parameter β for all results shows values higher than 1 due to the fact that the probability of Energiesbreakdown 2018, 11 occurrence, x FOR PEER increases REVIEW with the applied voltage. 6 of 13

0.95 PP 0.95 PP 0.9 HDPE 0.9 LDPE HP1 LP1 0.75 0.75 HP2 LP2 HP3 LP3 0.5 0.5 Probability Pro b0.25 ab ility 0.25

0.1 0.1

0.05 0.05 2 2 Breakdown Strength (kV/mm) 10 Breakdown Strength (kV/mm) 10

(a) (b)

FigureFigure 3. Weibull3. Weibull probability probability curves curves of breakdown of breakdown strength strength of (a )of HDPE/PP (a) HDPE/PP blends; blends; (b) LDPE/PP (b) LDPE/PP blends. blends.

ForFor the HDPE/PP samples, samples, the the range range of of breakd breakdownown strength strength was was 57 57 to to 80 80 kV. kV. From the shape parameter results, results, it it was was observed observed that that HP2 HP2 exhibited exhibited the minimum minimum ββ value,value, which which was was 3.5, 3.5, followed followed byby HP2 and HP3, with values of 4 andand 4.6,4.6, respectively.respectively. For the LDPE/PPLDPE/PP samples, samples, the the range range of of breakdownbreakdown strength strength was was 57 57 kV kV to to 85 85 kV. kV. From the shape parameter results, it was observed that that LP3 LP3 displayeddisplayed the the minimum minimum ββ value,value, which which was was 3, 3, followed followed by by LP2 LP2 and and LP1, LP1, with with values values of of 4 4 and and 5, 5, respectively.respectively. ComparingComparing the the breakdown breakdown strength strength of pureof pure and mixedand mixed polymer polymer samples, samples, for the blendedfor the blendedsamples samples of HDPE/PP, of HDPE/PP, the breakdown the breakdown strength ofstrength PP was of the PP lowest. was the The lowest. value wasThe fromvalue 10 was to 20from kV 10lower kV thanto 20 thatkV oflower HDPE than and that of theof HDPE blended and specimens. of the blended It can be specimens. noticed that It can the breakdownbe noticed that strength the breakdownof PP was enhanced strength withof PP the was addition enhanced of HDPE,with the which addition means of HDPE, that the which breakdown means strengththat the breakdown of HDPE is strengthlower. In of addition, HDPE theis lower. comparison In addition, between the blended comparison samples between did not blended show significant samples changesdid not amongshow significantthem when changes the blended among samples them when were comparedthe blended to samples the pure were samples. compared to the pure samples. For the breakdown strength of the LDPE/PP blends and of the corresponding pure materials, the blendedTable 1. samples Values displayedof the scale values and that shape were betweenparameters those for of PPthe andLow-density those of LDPE.polyethylene This shows that(LDPE)/polypropylene adding LDPE to PP increased(PP) blends. the breakdown strength of PP, but increasing the amount of PP decreased the breakdown strength of LDPE. The breakdown strength in the blended materials was Number of Scale Parameter, Shape Parameter, proportional to theSample ratio of the mixture. On the contrary, for the pure polymer samples, there was a considerable difference in the breakdownTests strength, e.g.,α the breakdown strengthβ of PP was 30 kV lower than that of HDPE. PP 9 57 8 LDPE 9 85 9 Table 1. Values of theLP1 scale and shape parameters9 for the Low-density 67 polyethylene 5 (LDPE)/polypropylene (PP) blends. LP2 9 69 4 LP3 9 62 3 Sample Number of Tests Scale Parameter, α Shape Parameter, β Table 2. ValuesPP of the scale 9 and shape parameters 57 for the High-density 8 polyethylene (HDPE)/polypropyleneLDPE (PP) blends. 9 85 9 LP1 9 67 5 LP2Number 9 of Scale 69 Shape Parameter, 4 Sample LP3 9Tests Parameter, 62 α β 3 PP 9 57 8 HDPE 9 73.6 7.7 HP1 9 80.5 4.6 HP2 9 66.5 3.5 HP3 9 72 4.6

For the breakdown strength of the LDPE/PP blends and of the corresponding pure materials, the blended samples displayed values that were between those of PP and those of LDPE. This shows that

Energies 2018, 11, 1448 7 of 13

Table 2. Values of the scale and shape parameters for the High-density polyethylene (HDPE)/ polypropylene (PP) blends.

Sample Number of Tests Scale Parameter, α Shape Parameter, β PP 9 57 8 HDPE 9 73.6 7.7 HP1 9 80.5 4.6 HP2 9 66.5 3.5 HP3 9 72 4.6

To interpret these results, it is useful to study the elements that affect the breakdown process of dielectric materials. The breakdown strength of dielectric materials is influenced by the degree of crystallinity and the formation of crystalline materials. This may be due to the electron scattering increment that correlates with the amount of crystalline volume. By increasing the degree of crystallinity, the breakdown strength of the polymers will decrease. The degree of crystallinity of LDPE is lower than that of HDPE, which may explain the higher breakdown strength of LDPE. However, the degree of crystallinity of PP is higher than those of HDPE and LDPE, resulting in the lowest value of breakdown strength for the PP samples [25]. In polymer insulation materials, the permeation of charge carriers (ionic or polar ), holes, and excess depends on their crystallinity. Mixing two different polymers with different ratios results in changes in the charge mobility behavior, which has a significant effect on the material conductivity [26]. At room temperature, polymers consist of both crystalline and amorphous phases. The hole activation energy of the amorphous structure is lower than that of the crystalline region, but the electron activation energy of the amorphous structure is higher than that of the crystalline structure. This means that the amorphous phase has a strong influence on the electron and hole activation and, hence, on the breakdown phenomena of polymers [14]. LDPE, HDPE, and PP have different molecular weights and distribution ranges. At room temperature, the weight of HDPE is higher than that of LDPE, while the molecule weight distribution of LDPE is wider than that of HDPE. This can be an additional reason of the higher breakdown strength of LDPE compared to PP and HDPE because the breakdown strength is dependent on the molecule size. The breakdown strength increases with the increment of the molecular size of polymers at room temperature [27,28]. However, impurity particles will be integrated into insulation materials during sample fabrication and manufacturing process. This affects the dielectric breakdown strength. These impurities modify the dielectric properties of the materials. Changing the permittivity and conductivity of polymers will influence their dielectric performance. In addition, the non-uniform conductivity leads to charge trapping development. The initiation of breakdown may be caused by the enhancement of the electric field due to this charge. Another possible justification for the reduction of the dielectric strength and the shape parameter of blended samples is the field improvement induced by the higher permittivity values of the processed dielectric samples. An additional reason is the existence of defects and impurities that cause weaknesses inside the blended material. The results of the breakdown strength experiments for the prepared samples have been discussed in this work on the basis of some aspects that have impact on the dielectric strength of insulation materials. The explanation of the dielectric breakdown of polymers needs a comprehensive understanding of the polymers essence, including their structure and physical and chemical properties. Also, there are numbers of processes that can cause the failure of polymers. These processes could work simultaneously, which makes it quite difficult to illustrate the development of the breakdown phenomenon. Because of this, many mechanisms are normally considered to interpret the measurement data. For insulation materials, their insulating characteristics will be modified when the temperature is varied because their natural properties, such as the dielectric properties, the loss factor, and the crystallinity, are influenced by the temperature. Moreover, a continuous increment of the insulation Energies 2018, 11, 1448 8 of 13 temperature will occur if the produced amount of heat losses is greater than the dissipated amount, resulting in decreased breakdown strength. In addition, the moisture content will affect the breakdown strength measurements by increasing the dielectric losses and surface conductivity. The humidity also influences the breakdown voltage of the materials by increasing the absorbed moisture on the surface. Hence, increasing chemical effects result in increasing the probability of flashover and surface discharge. Furthermore, by increasing the voltage rate increment, to the breakdown strength will increase.Energies This 2018, is 11 because, x FOR PEER the REVIEW material temperature increment is time-dependent and will influence8 of 13 the thermalEnergies breakdown 2018, 11, x FOR mechanism PEER REVIEW [29 ]. 8 of 13 will increase. This is because the material temperature increment is time-dependent and will influencewill increase. the thermal This is breakdown because the mechanism material [29].temperature increment is time-dependent and will 3.2. Differential Scanning Calorimetry (DSC) Results influence the thermal breakdown mechanism [29]. 3.2.The Differential DSC heating Scanning and Calorimetry cooling results (DSC) of Results the LDPE/PP and the HDPE/PP blends are presented in 3.2. Differential Scanning Calorimetry (DSC) Results Figures4 Theand DSC5, respectively. heating and cooling Melting results is an of endothermic the LDPE/PP mechanism, and the HDPE/PP in which blends the are polymer presented crystals in absorbFigures heatThe 4 to DSCand melt, 5,heating respectively. as can and be cooling clearly Melting results seen is an of in endothermic the Figure LDPE4./PP Crystallization mechanism, and the HDPE/PP in which is an blends exothermicthe polymer are presented mechanism,crystals in in whichabsorbFigures the heat 4 heat and to ismelt,5, givenrespectively. as offcan frombe clearly Melting the polymers seen is an in endothermicFigure when 4. theyCrystallization mechanism, crystalize, is asin an shownwhich exothermic the in Figurepolymer mechanism,5. Thecrystals melting in and crystallizationwhichabsorb the heat heat to melt,is temperatures given as canoff frombe clearly ofthe the polymers seen PP, in LDPE, Figure when HDPE, they4. Crystallization crystalize, LDPE/PP, as isshown and an exothermic HDPE/PP in Figure mechanism,5. blends The melting are in shown andwhich crystallization the heat is given temperatures off from the of thepolymers PP, LDPE when, HDPE, they crystalize, LDPE/PP, as and shown HDPE/PP in Figure blends 5. The are melting shown in Table3. From the DSC heating curves, the polymer melting temperature T m is the peak temperature. and crystallization temperatures of the PP, LDPE, HDPE, LDPE/PP, and HDPE/PP blendsm are shown The measuredin Table 3. PP From endothermic the DSC meltingheating curves, peak was theat polymer 170 ◦C, melting whereas temperature those of LDPE T is and the HDPE peak were temperature.in Table 3. FromThe measured the DSC PP heating endothermic curves, melting the polymer peak was melting at 170 °C, temperature whereas those Tm ofis LDPEthe peak and around 113 ◦C and 129 ◦C, respectively. The LDPE/PP and the HDPE/PP compounds showed two HDPEtemperature. were around The measured 113 °C PPand endothermic 129 °C, respectively. melting peak The wasLDPE/PP at 170 and°C, whereas the HDPE/PP those ofcompounds LDPE and dipsshowedHDPE symmetrical were two arounddips to the symmetrical 113 melting °C and peaks to 129 the points°C, melting respectively. of thepeaks pure pointsThe polymers. LDPE/PP of the Thepureand measuredthepolymers. HDPE/PP crystallizationThe compounds measured peak ◦ ◦ ◦ temperaturecrystallizationshowed two of PP, dips peak LDPE, symmetrical temperature and HDPE toof PP,the was LDPE,melting 119 andC, peaks 101HDPE pointsC, was and 119of 107 the°C, 101C,pure respectively.°C, polymers. and 107 °C, The respectively. measured crystallization peak temperature of PP, LDPE, and HDPE was 119 °C, 101 °C, and 107 °C, respectively. pp pp pp pp HDPE LDPE Exo Exo HDPE LDPE Exo Exo HP1 LP1 HP1 LP1

Heat flow Heat HP2 LP2 Heat flow

Heat flow Heat HP2 LP2 Heat flow HP3 LP3 HP3 LP3

60 80 100 120 140 160 180 60 80 100 120 140 160 180 60 80 100Temperature120 (°C)140 160 180 ° 60 80 Temperature100 120 ( C)140 160 180 Temperature (°C) (a) Temperature(b) (°C) (a) (b) Figure 4. Differential scanning calorimetry (DSC) heating thermograms showing the melting Figure 4. Differential scanning calorimetry (DSC) heating thermograms showing the melting temperatureFigure 4. Differential of (a) LDPE/PP scanning blends; calorimetry (b) HDPE/PP (DSC) blends. heating thermograms showing the melting temperature of (a) LDPE/PP blends; (b) HDPE/PP blends. temperature of (a) LDPE/PP blends; (b) HDPE/PP blends.

pp pp pp pp Exo Exo LDPE Exo Exo HDPE LDPE HDPE LP1 HP1 Heat flow Heat Heat flow Heat LP1 HP1 Heat flow Heat Heat flow Heat LP2 HP2 LP2 HP2 LP3 HP3 LP3 HP3 60 80 100 120 140 160 180 60 80 100 120 140 160 180 Temperature (°C) 60 80 100 120 140 160 180 60 80 Temperature100 120 (°C)140 160 180 Temperature (°C) Temperature (°C) (a) (b) (a) (b) Figure 5. DSC cooling thermograms showing the crystallization temperature of (a) LDPE/PP blends; Figure 5. DSC cooling thermograms showing the crystallization temperature of (a) LDPE/PP blends; Figure(b) 5.HDPE/PPDSC cooling blends. thermograms showing the crystallization temperature of (a) LDPE/PP blends; (b) HDPE/PP blends. (b) HDPE/PP blends. In Table 3, all results are displayed with two values for Tm and TC. The highest values correspondedIn Table to3, theall PPresults phase, are followed displayed by HDPE with andtwo finally values LDPE. for TAllm andof them TC. wereThe nearhighest the valuesvalues ofcorresponded the pure materials. to the PP This phase, means followed that the by blendsHDPE andin which finally PP LDPE. was added All of themto HDPE were and near LDPE the values were subjectof the pure to three materials. melt solidification This means thatmechanisms. the blends The in whichfirst one PP corresponded was added to toHDPE the crystallization and LDPE were of PP,subject and tothe three others melt to solidificationthe crystallization mechanisms. of LDPE The and first HDPE. one Thecorresponded results of LDPE to the andcrystallization HDPE in the of PP, and the others to the crystallization of LDPE and HDPE. The results of LDPE and HDPE in the

Energies 2018, 11, 1448 9 of 13

In Table3, all results are displayed with two values for T m and TC. The highest values corresponded to the PP phase, followed by HDPE and finally LDPE. All of them were near the values of the pure materials. This means that the blends in which PP was added to HDPE and LDPE were subject to three melt solidification mechanisms. The first one corresponded to the crystallization of PP, and the others to the crystallization of LDPE and HDPE. The results of LDPE and HDPE in ◦ the blended samples showed a small reduction in Tm, which was around 2 C, indicating a lessening in crystallite amount and possible co-crystallization [30]. For the HDPE/PP blend, only a single crystallization peak was noticed. This might be due to the overlapping of the peaks. This result indicates that HDPE and PP crystallized around the same temperature zone. The percent of crystallinity was calculated by dividing the heat of fusion normalized by the weight with the enthalpy of 100% crystalline polymer material. The measured heat of fusion of PE and PP was 293.6 J/g and 207.1 J/g, respectively. The degrees of crystallinity of HDPE, LDPE, and PP in the pure samples were 65%, 33%, and 47%, respectively. It is further noticed that the enthalpies of fusion of HDPE, LDPE, and PP were reduced in the blended samples compared to the pure samples, indicating a lower crystallinity development for each component. In order to show the nucleation effect of the blended materials on their crystallization, the crystallization onset temperature during cooling was determined for PP, LDPE, and HDPE. The onset temperature of pure PP was 123 ◦C, and for the samples containing LDPE was 124 ◦C, 125 ◦C, and 125.3 ◦C, thus slightly higher than those of pure PP. In the case of HDPE/PP, the crystallization onset temperature was between 118 ◦C and 119 ◦C, which confirms that HDPE has a greater effect on PP crystallization than LDPE [26,31,32].

Table 3. Crystallization temperatures of the LDPE/PP and HDPE/PP blends.

Degree of Melting Enthalpy (J/g) Tm1 Tm2 TC1 TC2 Crystallinity (%) Sample (◦C) (◦C) (◦C) (◦C) HDPE LDPE PP HDPE LDPE PP HDPE 129 - 117.1 - 190 - - 65 - - LDPE 114 - 101.7 - - 101 - - 34 - PP 171 - 119.4 - - - 99 - - 47 LP1 113 169 101.9 121.9 - 95 93 - 32 45 LP2 113.2 169.7 101.6 120.7 - 97 96 - 33 46 LP3 114.8 167.2 102.4 116.2 - 95 85.6 - 32 41 HP1 127.2 168.3 117.9 - 169.2 - 86.2 57 - 41 HP2 127.9 167.6 117.6 - 163.64 - 84 55 - 40 HP3 127 168 117.7 - 152.78 - 79 52 - 38

It is important to note that the dielectric strength of polymers can be affected by the degree of crystallinity [25]. Our results indicate that the combination of LDPE and HDPE in PP resulted in a small change of their degree of crystallinity. Accordingly, this small modification may not be sufficient to influence the dielectric properties. In addition, all pure and blended samples displayed an identical melting curve with a similar dip of the crystallization temperature, indicating a structure with similar size lamellae. The existence of wider lamellae has a significant impact on polymers conductivities. Moreover, the microstructure of the polymer matrix affects the mobility of charge carriers such as electrons, holes, and polar species. Also, the mixing of polymers in small amounts will not change the total crystallinity of the material but lead to distinct changes in the nanostructure. This will result in charge trapping and then reduce the mobility of charge carriers [10].

3.3. Dielectric Properties Results 0 Figure6a,b show the calculated real values of εr as frequency-dependent for the HDPE/PP and LDPE/PP samples. Referring to these figures, for each compound, the permittivity 0 of each sample did not show a discrepancy in relation to frequency, and εr of each material showed similar characteristic with different values. At a lower frequency, the permittivity value was slightly Energies 2018, 11, 1448 10 of 13 higher. However, increasing the applied frequency reduced the permittivity. The permittivity was high at lower frequencies as a result of Maxwell–Wagner polarization, which is also understood as a build-up of charges [33,34]. The polarization distinguishes the insulation materials and their permittivity values. The space charge polarization decreases the value of permittivity. Dipolar groups become stronger with the increase of the applied field frequency in order to regulate themselves. Thus,Energies this2018, lessens 11, x FOR the PEER impact REVIEW of dipolar groups on the permittivity [35]. Furthermore, the obtained10 of 13 0 results indicated that εr of the compound samples was greater than that of the pure materials. This was duematerials. to the factThis that was the due mobility to the offact charged that the groups mobility was of easier charged in the groups blended was materials easier in because the blended of the impuritiesmaterials because included of during the impurities sample fabricationincluded during which sample worked fabrication as conductive which particles. worked The as comparisonconductive betweenparticles. theThe HDPE/PP comparison blends between and the the HDPE/PP LDPE/PP bl blendsends and indicated the LDPE/PP that the blends HDPE/PP indicated blends that hadthe higherHDPE/PP permittivity blends had influenced higher permittivity by the degree influenced of crystallinity by the ofdegr HDPE,ee of whichcrystallinity was higher of HDPE, than thatwhich of LDPEwas higher [36]. than that of LDPE [36].

PP LDPE LP1 LP2 LP3 PP HDPE HP1 HP2 HP3 2.6 2.6 ) ) ε′ ε′ 2.5 2.5

2.4 2.4

2.3 2.3 Realpermittivity ( 2.2 Realpermittivity 2.2(

2.1 2.1 2 3 4 5 2 3 4 5 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz)

(a) (b)

FigureFigure 6.6. PermittivityPermittivity realreal componentcomponent ((εε0’)) ofof ((aa)) LDPE/PPLDPE/PP blends; (b) HDPE/PP blends. blends.

The measurement of the imaginary part of the relative permittivity, ɛ ′, is presented in Figure The measurement of the imaginary part of the relative permittivity, ε 00 , is presented in Figure7a,b, 7a,b, as a frequency-dependent parameter for the HDPE/PP and LDPE/PPr compounds. The results as a frequency-dependent parameter for the HDPE/PP and LDPE/PP compounds. The results showed showed that the values of ɛ ′ increased at frequencies higher than 3 kHz for all samples. This was that the values of ε 00 increased at frequencies higher than 3 kHz for all samples. This was attributable attributable to the rincreased in relaxation loss of the dielectric materials when they were exposed to to the increased in relaxation loss of the dielectric materials when they were exposed to high-frequency high-frequency field. Furthermore, ionic polarization may possibly occur in a higher-frequency field. Furthermore, ionic polarization may possibly occur in a higher-frequency range, leading to range, leading to additional increments of the ɛ ′ values. This may induce the charge carriers to additional increments of the ε 00 values. This may induce the charge carriers to immediately cross the immediately cross the surfacer of the polymer compound at higher frequencies [37]. Between 1 kHz surface of the polymer compound at higher frequencies [37]. Between 1 kHz and 3 kHz, the value of and 3 kHz, the value of ɛ ′ was low because of slower polarization mechanisms [38]. From 40 Hz to 00 00 εr was low because of slower polarization mechanisms [38]. ENREF_35. From 40 to 1 kHz, εr value 1 kHz, ɛ ′ value declined, which was also due to slower polarization mechanism. This was due to declined, which was also due to slower polarization mechanism. This was due to the fact that long the fact that long carbon chains attached to the main chain were stretching and twisting. From the carbon chains attached to the main chain were stretching and twisting. From the observation of observation of these results, the ɛ ′ values of pure LDPE and HDPE samples were lower than that 00 these results, the εr values of pure LDPE and HDPE samples were lower than that of the PP sample. of the PP sample. However, the ɛ ′ values of the blended samples were decreased compared to However, the ε 00 values of the blended samples were decreased compared to those of the pure samples those of the purer samples at a low frequency. These results may explain the lower dielectric strength at a low frequency. These results may explain the lower dielectric strength of polymer compounds in of polymer compounds in comparison with LDPE and HDPE samples. This is due to a higher comparison with LDPE and HDPE samples. This is due to a higher possibility of creating conduction possibility of creating conduction paths and subsequently to a reduction of the breakdown voltage. paths and subsequently to a reduction of the breakdown voltage.

)

) 0.08 0.08 PP HDPE HP1 HP2 HP3 PP LDPE LP1 LP2 LP3 ε′′ ε′′ 0.07 0.07

0.06 0.06 (b) 0.05 (a) 0.05

0.04 0.04 0.03 0.03

0.02 0.02

Imaginarypermittivity ( 0.01

Imaginarypermittivity ( 0.01

0 0 2 3 4 5 2 3 4 5 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz) (a) (b)

Figure 7. Permittivity imaginary component (ε’’) of (a) LDPE/PP blends; (b) HDPE/PP blends.

Energies 2018, 11, x FOR PEER REVIEW 10 of 13 materials. This was due to the fact that the mobility of charged groups was easier in the blended materials because of the impurities included during sample fabrication which worked as conductive particles. The comparison between the HDPE/PP blends and the LDPE/PP blends indicated that the HDPE/PP blends had higher permittivity influenced by the degree of crystallinity of HDPE, which was higher than that of LDPE [36].

PP LDPE LP1 LP2 LP3 PP HDPE HP1 HP2 HP3 2.6 2.6 ) ) ε′ ε′ 2.5 2.5

2.4 2.4

2.3 2.3 Realpermittivity ( 2.2 Realpermittivity 2.2(

2.1 2.1 2 3 4 5 2 3 4 5 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz)

(a) (b)

Figure 6. Permittivity real component (ε’) of (a) LDPE/PP blends; (b) HDPE/PP blends.

The measurement of the imaginary part of the relative permittivity, ɛ ′, is presented in Figure 7a,b, as a frequency-dependent parameter for the HDPE/PP and LDPE/PP compounds. The results showed that the values of ɛ ′ increased at frequencies higher than 3 kHz for all samples. This was attributable to the increased in relaxation loss of the dielectric materials when they were exposed to high-frequency field. Furthermore, ionic polarization may possibly occur in a higher-frequency range, leading to additional increments of the ɛ ′ values. This may induce the charge carriers to immediately cross the surface of the polymer compound at higher frequencies [37]. Between 1 kHz and 3 kHz, the value of ɛ ′ was low because of slower polarization mechanisms [38]. From 40 Hz to 1 kHz, ɛ ′ value declined, which was also due to slower polarization mechanism. This was due to the fact that long carbon chains attached to the main chain were stretching and twisting. From the observation of these results, the ɛ ′ values of pure LDPE and HDPE samples were lower than that of the PP sample. However, the ɛ ′ values of the blended samples were decreased compared to those of the pure samples at a low frequency. These results may explain the lower dielectric strength of polymer compounds in comparison with LDPE and HDPE samples. This is due to a higher Energies 2018, 11, 1448 11 of 13 possibility of creating conduction paths and subsequently to a reduction of the breakdown voltage.

)

) 0.08 0.08 PP HDPE HP1 HP2 HP3 PP LDPE LP1 LP2 LP3 ε′′ ε′′ 0.07 0.07

0.06 0.06 (b) 0.05 (a) 0.05

0.04 0.04 0.03 0.03

0.02 0.02

Imaginarypermittivity ( 0.01

Imaginarypermittivity ( 0.01

0 0 2 3 4 5 2 3 4 5 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz) (a) (b) Figure 7. Permittivity imaginary component (ε”) of (a) LDPE/PP blends; (b) HDPE/PP blends. Figure 7. Permittivity imaginary component (ε’’) of (a) LDPE/PP blends; (b) HDPE/PP blends. 4. Conclusions

In this work, breakdown strength measurements of polymer compounds were successfully performed to investigate the impact of synthesis polymers on their dielectric properties. Impedance measurements as a function of frequency and the average degree of crystallinity taken from differential scanning calorimetry (DSC) measurements were successfully determined to characterize the LDPE/PP and HDPE/PP compounds. It was observed that the dielectric strength of the LDPE/PP and HDPE/PP compounds were lower than those of the pure LDPE and HDPE samples. However, these values revealed an enhancement of the dielectric strength for PP when HDPE and LDPE were added in different ratios. The dielectric constants of the blended materials exhibited an increasing trend compared to the pure samples, as observed for the dielectric losses at low frequency, which had higher values than for the pure samples. The DSC results displayed two exothermic crystallization peaks, representing two crystalline phases. In addition, the DSC results showed that the blended samples were physically bonded, and no occurrence of co-crystallization was detected in the polymer compounds.

Author Contributions: S.Z.A.D., H.A.I., and B.C.A. conceived and designed the research methodologies; S.Z.A.D. and M.Z.H.M. performed the experiments and analyzed the data; S.Z.A.D., N.A.A.L., H.A.I., and B.C.A. wrote the . Funding: The authors thank the University of Malaya, Malaysia and Ministry of Higher Education, Malaysia for supporting this work through HIR research grant (grant number: HIR H-16001-D00048), Postgraduate Research Funds (grant number: PPP PG028-2016A) and Fundamental Research Grant Scheme (grant number: FRG0413-SG-1/2015). Conflicts of Interest: The authors declare no conflict of interest.

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